Don't be too sure about that. Take a look at points 3 & 4 here.
I won't try to argue that the market hasn't changed over the years. Flashy graphics and instant gratification are certainly in favour these days. I just don't think age has as much to do with it as you might think.
It seems to me that it's more due to the fact that while the population of gamers is growing exponentially, the population of flight simming gamers is not growing nearly as much, if at all. It's a damned shame really but at least we have people like TK and Oleg Maddox keeping the genre alive.